The present invention relates generally to controlling striations in lamps powered by electronic ballasts.
More particularly, this invention pertains to methods for controlling lamp striations by supplying the lamp with asymmetric lamp currents.
Methods of controlling lamp striations are known in the art. For example, U.S. Pat. Nos. 5,041,763 and 5,192,896 teach the use of methods that include the step of supplying DC or low frequency AC current to a lamp in order to control striations. The primary disadvantage of these methods is that they require electronic ballasts that include additional components for generating the DC or low frequency AC currents and these additional components increase the costs and losses associated with the electronic ballasts.
U.S. Pat. No. 6,465,972 also teaches a method of controlling striations. In this patent, striations are controlled by varying the amplitude of the current that is supplied to the lamp using an amplitude modulation circuit. This method suffers from the same disadvantage as the method taught by the '763 patent. That is, it requires an electronic ballast that includes additional components and those components increase the costs and losses associated with that ballast.
U.S. Pat. No. 6,069,453 teaches a method of controlling striations by supplying a lamp with a current that includes a DC component, a high frequency AC component, and a low frequency component. In addition, the amplitude of the high frequency AC component must be at least 500 times higher than the amplitude of the low frequency component. As was the case with the '763, '896, and '972 patents, this method requires the use of an electronic ballast having additional components that increase the costs and losses associated with the ballast.
U.S. Pat. Nos. 5,701,059, 5,369,339, and 5,001,386 all teach methods of controlling striations in lamps. All of these methods, however, require the use of additional components that are not required to perform the functions most commonly performed by an electronic ballast. For example, the method taught by the '059 patent requires the use of an impedance that is connected in parallel with a lamp. The method taught by the '339 patent requires a DC device that can supply DC current to a lamp. And, the method taught by the '386 patent requires a back end rectifier that is used to supply a DC current to a lamp. The requirement of these additional components increases the cost and losses associated with the electronic ballasts required to implement these methods.
What is needed, then, is a method of controlling striations in a lamp that does not require the use of an electronic ballast that includes additional components that increase the costs and losses associated with that ballast.